Download the srb2_3ds archive from the latest release on Github and extract the files to /3ds/srb2_3ds on your SD card.
Download SRB2's game files here or from srb2.org and extract them to /3ds/srb2_3ds as well. You may also copy the files to the root directory, but make sure config.cfg is located in the same directory as the game files.

Only New3DS is supported at the moment.
Please check the repo's README.